The activation of hydrocarbon synthesis catalysts is widely reported in the literature. Activation usually involves reduction, in a flowing hydrogen or hydrogen containing gas stream, at elevated temperatures. The reduction proceeds from a reducible metal compound, usually, but not necessarily the metal oxide. Thus, the literature reports reduction of metal oxides to the elemental or active form of the metal as well as reduction from metal compounds, impregnated either as water soluble or hydrocarbon soluble salts.
For example hydrocarbon synthesis catalysts are usually prepared by depositing a metal, such as cobalt or other Group VIII metals onto a support, by techniques such as impregnation, the metal being in a water soluble (e.g., cobalt nitrate) or hydrocarbon soluble (e.g., cobalt carbonyl) form. The catalyst is usually dried to eliminate the solvent, but reduction (or activation) does not occur during the drying step even though a hydrogen containing gas may be used as the drying agent.
Once having the metal compound impregnated onto the support, activation by reducing to the elemental or catalytically active form of the metal may proceed directly from the metal compound, or the metal compound may be converted to a more easily reducible form, such as the oxide and then reduced to its elemental or catalytically active form. In either case the metal compound must be reducible to its catalytically active form; the oxide is one form of reducible metal compound.
U.S. Pat. No. 4,605,676, shows the activation of cobalt containing hydrocarbon synthesis catalysts wherein the activation steps, including oxidation and reduction, are all carried out at temperatures between about 100.degree. C. and 450.degree. C. U.S. Pat. Nos. 4,670,675 and 4,605,679 also show activation procedures wherein cobalt containing catalysts are reduced at temperatures below about 500.degree. C. These patents, however, do not recognize the effect of water partial pressure when water is produced during the reduction (or activation) step. Thus, this invention recognizes that temperature of reduction, by itself, does not fully account for differences in hydrocarbon synthesis catalyst activity.
Additionally, the aforementioned patents as well as U.S. Pat. No. 4,801,573, conduct the reduction step at ambient conditions, that is, one atmosphere, and the water partial pressure will necessarily be quite low. Obviously, the water partial pressure cannot be greater than the total pressure of the system. These reducing steps reflect laboratory experiments and are representative of gross effects such as oxidation or reduction. Nevertheless, large scale or commercial operations necessarily involve reducing the metal compound at elevated pressures because of pressure drop considerations in large-scale equipment.
Recently allowed European patent 0168894B1 discloses an activation process wherein the hydrogen partial pressure during catalyst activation is gradually raised to a value at least five times the initial hydrogen partial pressure at treatment temperatures of 200.degree.-350.degree. C. The total pressure during catalyst activation never exceeded 2 bar. Such conditions do not even approach the conditions at which commercial activation procedures take place, and consequently, do not afford a situation where the effects of water partial pressure can be determined.
Consequently, laboratory scale experiments at atmospheric or relatively low pressures can provide no guidance for water partial pressure effects at elevated pressures. Furthermore, at ambient conditions, problems in catalyst activity due to water partial pressure effects will not be recognizable since at total pressures of one or two atmospheres water partial pressure does not present a problem. Further, the recognition of water partial pressure as an important element in hydrocarbon synthesis catalyst activation has never before been recognized.
One object of this invention, therefore, is providing a method for activating hydrocarbon synthesis catalysts at pressures indicative of commercial operation and greater than atmospheric pressure. Another object of this invention is providing a procedure for determining the water partial pressure by determining the factors that affect water pressure. This procedure allows the prediction of water partial pressure under a variety of interdependent variables, and therefore, allows the pre-determination of a number of different conditions that allow an operator to maximize hydrocarbon synthesis catalyst activity.